1. Field of Invention
This invention relates to electrochemical analytical systems and more particularly to continuous, flow independent, monitoring of sample streams for the total concentration of a series of ionic species.
2. Prior Art Statement
The present invention relates to the flow independent monitoring of the total concentration of a series of ionic species of the form X.sup.-n, H.sub.m X.sup.-n+m where m varies from 1 to n and X.sup.-n is an anion selected from the group consisting of fluoride ion F.sup.-, and sulfide ion S.sup.-2. For example, the measurement of total sulfide concentration, that is H.sub.2 S, HS.sup.-, and S.sup.-2 has long been an important factor in the treatment of industrial process streams. Sewage treatment plants, pulp and paper manufacturers, and textile producers test for total sulfide content in effluent streams to protect pipes from corrosion, and to limit noxious ordors. Further attention was given to these species by the federal standards for industrial air which were promulgated on May 29, 1971 in Federal Register Vol. 36, No. 105, regulating the allowable amounts of hydrogen sulfide, and hydrogen fluoride in workroom air. Continuous monitoring of total fluoride is important in the plating and transistor manufacturing industries where the make-up of rinse baths are continually checked. Municipal and bottled water plants also need to make continuous measurements of total fluoride, that is, HF and F.sup.-, in their streams.
Electrodes specific to fluoride ion F.sup.-, and sulfide ion S.sup.-2, have been known in the literature and in the marketplace. U.S. Pat. No. 3,431,182 issued Mar. 4, 1969 to Frant et al. disclosed a fluoride ion sensitive electrode. U.S. Pat. No. 3,672,962 issued June 27, 1972 to Frant et al. disclosed a sulfide ion sensitive electrode. Neither of the above electrodes measure the total concentration of the anion together with the concentration of its protonated species.
An alternate approach to measuring H.sub.2 S was disclosed by Riseman et al. in U.S. Pat. No. 3,915,831, issued Oct. 28, 1975. A fluoride electrode is used as a reference electrode in conjunction with a sulfide ion sensitive electrode. The reference electrode can also be a pH glass type electrode or a sodium sensitive glass type electrode, but if these latter two are used, pH must be carefully controlled. The present invention contemplates the use of a pH electrode not as a reference, but as a H.sup.+ ion sensitive device, in contradistinction to the Riseman disclosure. The use of a buffer system is eliminated.
Two patents issued to Riseman et al. describing a method, and an apparatus, respectively, for electrochemical monitoring employ the use of a tag ion and a tag ion electrode as an alarm system. U.S. Pat. No. 3,846,257 issued Nov. 5, 1974 and U.S. Pat. No. 3,964,988 issued June 22, 1976. A reagent stream containing a selected amount of tag ion and ionic species of interest is added to the sample stream. The mixed stream is monitored by two electrodes sensitive to the tag ion and the ion of interest, and a potential difference between the two electrodes is measured. If the sample stream begins to add an amount of the ionic species of interest to the mixed stream, the ratio of concentrations of tag ion to ionic species of interest is altered, the potential difference is altered, and the alarm is triggered. The present invention does not rely on the addition of a reagent stream containing a fixed ratio of tag ion and species of interest to the sample stream.
Various methods have been proposed for adjusting the pH of a sample stream. Diggens disclosed a method in U.S. Pat. No. 4,131,428, issued Dec. 26, 1978, that does not introduce interferring, reactive, or test ions into the sample stream. This method is preferably used in the present invention to adjust the pH of the sample stream to required levels.